USB ports will be in default in 97.9954646546% of the PCs, nowadays even ethernet/LAN card comes in default.
LAN crad costed only rs. 175 (this feb when I got mine; Realtek)
So you can go for USB/LAN modem depending on your requirements and what you have in your PC installed.
Regarding modem purchase, visit:
http://www.bsnl.co.in/service/dataon...ht%20purchase: